1. Field of the Invention
The present invention relates to a magneto-resistance effect element and a sensor.
2. Description of the Related Art
Sensors using magneto-resistance effect (MR) elements have sometimes been used to sense the angle of rotation of a steering wheel in a vehicle (see Japanese Patent Application Laid-open No. 2006-29792A, Japanese Patent Application Examined Publication No. 7-119619B, Japanese Patent No. 3017061, and “Development of Highly Sensitive Terrestrial Sensor using TMR Junctions”, Kou, F. et al., Journal of the Magnetics Society of Japan Vol. 32, No. 3, 2008, published by The Magnetic Society of Japan). Specifically, the sensor is configured such that for example, a magnet is mounted on the shaft of the steering wheel and such that a sensing circuit including a Wheatstone bridge with a plurality of MR elements connected to each other is fixed at a position opposite to the magnet. When a driver turns the steering wheel, the magnet mounted on the shaft of the steering wheel is moved to change the position of the magnet relative to each of the MR elements forming the Wheatstone bridge. Then, a magnetic force exerted on each MR element by the magnet changes, and the change in magnetic force in tune changes the resistance value of each MR element. Thus, the sensor can operate as follows. The sensor senses the change in the resistance value of each MR element to determine the direction and amount of movement of the magnet. Based on the determined direction and amount, the sensor determines the direction in which and the angle by which the steering wheel has been turned. Then, based on the amount of rotation of the steering wheel sensed by such a sensor, a motor configured to assist in the movement of a steering axle is driven and is controlled. Thus, so-called power steering, which requires only a weak force, can be carried out.
Examples of an MR element include a CIP-GMR (Current in Plane-Giant Magneto-resistance effect) element, a CPP (Current Perpendicular to Plane)-GMR element, a TMR (Tunnel Magneto-resistance effect) element, and an AMR (Anisotropic Magneto-resistance effect) element; the CIP-GMR element is a giant magneto-resistance effect element including paired ferromagnetic layers and a nonmagnetic layer positioned between the ferromagnetic layers as an intermediate layer and in which a sense current flows parallel to the layers, the CPP-GMR element is a GMR element in which a sense current flows perpendicularly to the layers, the TMR element is a element including an insulating layer instead of the nonmagnetic layer as an intermediate layer and in which a sense current flows perpendicularly to the layers, and the AMR element utilizes an anisotropic magneto-resistance effect.
A general GMR element includes a pinned layer that is a ferromagnetic layer shaped like a pillar with a desired size and having a fixed magnetization direction, a free layer that is a ferromagnetic layer having a magnetization direction varied in response to external magnetic fields, and a nonmagnetic intermediate layer sandwiched between the pinned layer and the free layer. The general TMR element is configured such that an insulating intermediate layer (barrier layer) is sandwiched between the pinned layer and the free layer. Such an MR element is also called a spin valve film (SV film). A cap layer is provided at the upper end of the MR element. An underlying layer is provided at the lower end of the MR element. The MR element is sandwiched between an upper electrode layer and a lower electrode layer. In the CPP-GMR element and the TMR element, a sense current flows orthogonally to the layers. An example of the GMR element is disclosed in Japanese Patent Application Laid-open No. 11-126933A. An example of the TMR element is disclosed in Japanese Patent Application Laid-open No. 2001-102659A.
As described above, some sensors are configured as follows. A plurality of MR elements are connected to each other to form a Wheatstone bridge. A sensing circuit including the Wheatstone bridge is fixed at a position opposite to a magnet mounted on a sense target member (for example, the shaft of the steering wheel). The sensor measures a plurality of output voltages from the Wheatstone bridge. There has been a demand to make the sensing accuracy of the sensor as high as possible.
The following matters are obstacles to an increased sensing accuracy. The magnetization direction of the pinned layer slightly changes in response to external magnetic fields, and a variation is not sensitive in the magnetization direction in the free layer in response to external magnetic fields. That is, the pinned layer is desirably prevented, as originally intended, from being affected by external magnetic fields and from having its magnetization direction varied. The free layer desirably finely detects weak external magnetic fields so that the magnetization direction in the free layer can be varied, as originally planned.